1. Field of the Invention
The invention relates to a method of producing three-dimensional bodies of metal which wholly or for selected parts consist of a composite of crystalline or nanocrystalline metal particles in a matrix of amorphous metal.
2. Description of the Related Art
When cooling a metallic material from melt to solid phase, a polycrystalline structure is usually obtained. Here, the microstructure consists of a large number of different grains where the atoms in each grain are arranged according to some kind of regular pattern. If the atoms instead are completely disordered and there are no grains with regularly positioned atoms, the material is said to be amorphous. This can for example be achieved by cooling a melt very rapidly so that there is no time for any grains to grow, or by very extensive mechanical deformation where the grains are disrupted.
At the beginning of the sixties, the first amorphous metals were produced by spraying a thin layer of melt onto a heat-conducting base. This resulted in very high cooling speeds of 105-106 K/s and there was no time for any grains to grow so the disordered structure was maintained also in the solid phase. However, the resulting alloys were very thin with a thickness of only some tens of micrometers and therefore had limited ranges of application.
Amorphous bulk metals or amorphous structural metals, i.e. amorphous metals with dimensions that permitted structural applications, were not produced until the seventies from specially composed alloys. Bulk metals of these alloys were produced by cooling from melt at a cooling speed of about 1000 K/s, but contained i.e. the expensive metal palladium, which prevented larger volumes of production. At the end of the eighties, Professor Inoue at the Tohoku University in Japan managed to develop various multi-component systems consisting of ordinary metallic elements which resulted in an amorphous bulk structure when cooling from melt. In the years that followed, a great number of different amorphous metal systems have been found. In the literature, these are often denominated “Bulk Metallic Glasses”.
Completely amorphous materials often have a very high hardness, whereas polycrystalline materials are more ductile. In many situations, there is a desire to combine properties of materials with amorphous structure and crystalline structure, respectively. In a polycrystalline structure, the area between the crystals, the grain boundaries, can be regarded as disordered or amorphous. If the number of grains is increased, i.e. the size of the grains is decreased, the degree of grain boundaries or disordered material in the structure is also increased. In a nanocrystalline material, the grains are very small and the material can be regarded as a composite material with many small crystals in a disordered (amorphous) matrix.
The composite materials of crystalline metal in an amorphous matrix have been produced by subjecting alloys, which can form amorphous metal by rapid cooling, to cooling from melt in accordance with a controlled time-temperature curve in such a manner that a growth of crystals has been allowed before the material has been cooled to such an extent that continued crystal growth has been stopped. U.S. Pat. No. 7,244,321 describes such a method, where a further element has been added to the alloy system so as to function as a growth promoter for the crystallization. It is also known to start from an object of an amorphous alloy and heat-treat the object in accordance with a controlled time-temperature curve causing a growth of crystals in the material and thereafter rapidly cool the object again. Production of composite materials of nanocrystalline particles in an amorphous matrix is for instance described in an article by Inoue et al: “Synthesis of High Strength Bulk Nanocrystalline Alloys Containing Remaining Amorphous Phase”, Journal of Metastable and Nanocrystalline Materials, Vol. 1, (1999), pp 1-8.
In the production of three-dimensional bodies, for instance constructional details which can have a considerable material thickness and varying geometrical shape, it is not possible to achieve a sufficiently well-defined cooling speed in all parts of the body with these known methods. Owing to the fact that the size and rate of growth of the crystals depend on the temperature and the time period during which the temperature in question acts on the material, these methods will not give a body with controlled material properties.